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Submitted on 9 Jan 2007
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hal-00123276, version 1 - 9 Jan 2007
MIMAC-He3 : MIcro-tpc MAtrix of Chambers of 3
He
D. Santos, O. Guillaudin, Th. Lamy, F. Mayet and E. Moulin Laboratoire de Physique Subatomique et de Cosmologie, CNRS/IN2P3 et Universit´e Joseph Fourier (Grenoble-1),
53, avenue des Martyrs, 38026 Grenoble cedex, France
The project of a micro-TPC matrix of chambers of 3He for direct detection
of non-baryonic dark matter is outlined. The privileged properties of3He are
highlighted. The double detection (ionization - projection of tracks) will as-sure the electron-recoil discrimination. The complementarity of MIMAC-He3 for supersymmetric dark matter search with respect to other experiments is illustrated.The modular character of the detector allows to have different gases to get A-dependence. The pressure degreee of freedom gives the possibility to work at high and low pressure. The low pressure regime gives the possibility to get the directionality of the tracks. The first measurements of ionization at very few keVs for3
He in4
He gas are described.
Keywords: Dark Matter, TPC, Helium-3, Spin-dependent interaction.
1. Introduction
other target nuclei. First, 3
He being a spin 1/2 nucleus, a detector made of such a material will be sensitive to the spin-dependent interaction, lead-ing to a natural complementarity to most existlead-ing or planned Dark Mat-ter detectors (ν telescopes, scalar direct detection as well as proton based spin-dependant detectors). In particular, it has been shown [17,18] that an
3
He based detector will present a good sensitivity to low mass ˜χ , within the framework of effective MSSM models without gaugino mass unification at the GUT scale [6,7].
Fig. 1. SUSY non minimal models, calculated with DarkSusy code [7]. In grey the models giving an axial cross section ( ˜χ-3He ) higher than the exclusion plot of
MIMAC-3He with 10kg [18]. These models are compared with exclusion plots of scalar
The3
He presents in addition the following advantages with respect to other sensitive materials for WIMPs detection :
- a very low Compton cross-section to gamma rays, two orders of magnitude weaker than in Ge : 9 × 10−1barns for 10 keV γ-rays
- the neutron signature made possible by the capture process : n +3
He → p +3
H + 764 keV Indeed it allows for an easy discrimination with ˜χ0
signal (E . 6 keV). This property is a key point for Dark Matter search as neutrons in underground laboratories are considered as the ultimate background.
Any dark matter detector should be able to separate a ˜χ event from the neutron background. Using energy measurement and electron-recoil dis-crimination, MIMAC-He3 presents a high rejection for neutrons due to capture and multi-scattering of neutrons [20]. The MIMAC project pro-pose a modular detector in which different gases (3
He , CF4 ) can be used
to have a dependence on the mass of the target. The 19
F is other good target nucleus choice to have the axial interaction open, but proton based, increasing the attractiveness of the detector.
The MIMAC detector has two different regimes of work: i) high pressure (1,2 or 3 bar) and ii) low pressure (100 - 200 mbar). These two regimes allow us to have Wimp events at high pressure and search for correlation with the galactic halo apparent movement at low pressure. This last possibility should be validated with a special read out electronics as an important step of the project.
2. Micro-TPC and ionization-track projection detection
The micro time projection chambers with an avalanche amplification using a pixelized anode presents the required features to discriminate electron -recoil events with the double detection of the ionization energy and the track projection onto the anode. In order to get the electron-recoil discrimination, the pressure of the TPC should be such that the electron tracks with an energy less than 6 keV could be well resolved from the recoil ones at the same energy convoluted by the quenching factor. The electrons produced by the primary interactions will drift to the amplification region (mesh) in a diffusion process following the well known distribution characterized by a radius of D ≃ λp(L[cm]) where λ is tipically 200 µm for3
can be obtained. The quenching factor is an important point that should be addressed to quantify the amount of the total recoil energy recovered in the ionization channel. No measurements of the quenching factor (QF) in
3
He have been reported. However, an estimation can be obtained applying the Lindhard calculations [11]. The estimated quenching factor given by Lindhard’s theory for 3
He shows up to 70 % of the recoil energy going to the ionization channel for 5 keV3
He recoil.
3. Source MIMAC
In order to measure the QF for3
He and4
He we have developed at the LPSC a dedicated facility producing very light ions at a few keV energies. This facility, called source MIMAC, incorporates an ECR ion source coupled to a Wien filter, selecting q/m, and a high voltage extraction going up to 50 kV.
Fig. 2. Time of flight measurements performed with the MIMAC source. The figure shows the spectra at the two different positions (close to and far from the interface (source-chamber)) used to measure the3
He+
The characterization of the output energies is made by a separate time of flight measurements as we can see on fig.2 for the case of3
He ions ac-celerated at 15 kV having a mean output energy of 3.7 keV. Using this facility we can explore the ionization at very low energies for3
He ions. We have measured by TOF, five output energies going from 13.7 keV up to 3.7 keV corresponding to five values from 30 to 15 kV of accelerating voltage extraction. Ionization measurements have been performed, with a standard micromegas grid in a gas chamber (95% of 4
He and 5% of isobutane at 1 bar). A linear calibration fits very well the points measured and extrapolat-ing to even lower voltage extraction, we can estimate the maximum output energy corresponding to 10.5 kV to 800 eV. On fig.3 the spectrum of the ionization left in the chamber by 3
He at 800 eV is shown. On the same spectrum we show an internal conversion electron spectrum of57
Co during the two minutes the beam of3
He was on. This57
Co source will allow us to get an idea of the equivalent electron energies. We can differentiate on the spectrum the peak of ionization well separated from the electronic noise.
Fig. 3. A two minutes spectrum showing ionization peak corresponding to a beam, produced by the MIMAC source, of3He at an energy estimated to 800 eV. An internal
conversion source of57Co spectrum is shown on the same spectrum. This source will
out electronics that is building up at our laboratory. References
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